In general, construction machine such as a bulldozer is provided with a blade. The primary modes of work for such construction machine are digging, earth moving, and earth removal; the angle of incline of the blade relative to the ground (hereinbelow called the “pitch angle”) is preferably set to a suitable angle in accordance with the work mode, in order to improve work efficiency.
With the purpose of reducing the burden on an operator, there has been proposed a technique for automatically setting the pitch angle in accordance with the work mode (hereinbelow called “auto-pitch control”) (see Japanese Laid-open Patent Publication 7-166575). In this publication, the pitch angle is detected by calculating the stroke of a blade tilt cylinder, on the basis of an amount of hydraulic oil supplied to the blade tilt cylinder from a main valve. The amount of hydraulic oil supplied to the blade tilt cylinder is calculated on the basis of the supply time and the opening area of a blade tilt cylinder valve constituting the main valve.